1. Field of the Invention
The present invention relates to a method for manufacturing a printed wiring board having conductors formed on upper and lower surfaces of a core substrate and a through-hole conductor formed in a penetrating hole of the core substrate to connect the conductors formed on the upper and lower surfaces.
2. Discussion of the Background
In Japanese Laid-Open Patent Publication 2006-41463, a penetrating hole is formed by using a laser to form a first opening portion from a first-surface side of a core substrate and also using a laser to form a second opening portion from a second-surface side. The first opening portion tapers as it goes toward the second surface, and the second opening portion tapers as it goes toward the first surface. Then, by filling plating in the penetrating hole, a small-diameter through-hole conductor is formed to connect the upper and lower surfaces of the core substrate. FIGS. 13(A), (B) show penetrating hole 128 filled with plating where two tapered opening portions (128a, 128b) are directly connected. As shown in FIG. 13(B), when through-hole conductor 136 is formed by filling plating in penetrating hole 128, where tapered opening portions (28a, 28b) are directly connected, stress tends to be concentrated in one location (128a′, 128b′) at which the diameter of through-hole conductor 136 reaches its minimum. The entire contents of this publication are incorporated herein by reference.